Drilling and cementing slim hole wells

ABSTRACT

A slim hole well is drilled using a drilling fluid comprising blast furnace slag and water which drilling fluid is circulated during the drilling to lay down a filter cake. Thereafter, an activator is added and generally, an alkaline material and additional blast furnace slag, to produce a cementitious slurry which is passed down a casing and up into an annulus to effect primary cementing.

BACKGROUND OF THE INVENTION

This invention relates to drilling and cementing slim hole boreholes.

Boreholes for applications such as oil wells are normally drilled withan initial diameter of about 20 inches. It has long been known to drillsmaller diameter holes, known as slim holes, for exploratory wells.These wells generally have a borehole of no more than 5-6 inches initialdiameter. It would be desirable to drill slim hole production wells insome instances.

As with the drilling of conventional boreholes, the slim hole process isgenerally carried out using a rotary drilling process. The rotarydrilling of a borehole is accomplished by rotating a drill string havinga drill pipe and a drill bit at its lower end. Weight is applied to thedrill string while rotating to create a borehole into the earth. Thedrill string is hollow and sections are added to drill string toincrease its length as the borehole is deepened. This rotary drillingprocess creates significant amounts of friction which produces heatalong with fragments of the strata being penetrated. The fragments ofthe strata must be removed from the borehole and the drill bit must becooled to extend its useful life. Both of these necessities areaccomplished by the circulation of a fluid down through the drill stringand up to the surface between the drill string and the wall of theborehole. As this done, a layer of solids is deposited on the boreholewall which is commonly referred to as filter cake. The filter cake isformed by the combination of solids in the drilling fluid and thedifferential between the fluid pressure in the borehole and theformations being penetrated. Since the pressure exerted by the fluidcolumn in the borehole is preferably slightly to significantly higherthan the pressure in the pores of the exposed formation, there is atendency for the liquid phase of the drilling fluid to leak off into theformation. As this occurs, the solids are deposited along the boreholewall since they are typically of sufficient size to prevent substantialpenetration into the formation.

Once the borehole has been drilled to the desired depth, it may bedesirable to isolate the separate areas, zones or formations transversedby the borehole. For extraction of fluids from formations, a conduit(casing) must be inserted into the borehole extending from the surfacedownward.

At this point it becomes necessary to fill the annulus between thecasing and the borehole wall with a material which will seal the annulusand provide structural support for the casing. This is commonly referredto as primary cementing.

Slim hole drilling creates two problems. First, it is more difficult toremove the fluid in the annulus between the borehole and casing becausethe annulus is smaller. Second, it is more difficult to remove thefilter cake on the side of the borehole wall, again because the annulusis so narrow. This latter problem is significant because the filter cakeis generally incompatible with the cement. This can result in channelingof fluids used to wash out the drilling fluid and/or channeling of thecement leaving significant areas of unremoved and incompatible drillingfluid in the annulus. This results in voids in the final cementing job.

It has long been known to use pozzolans which broadly encompass, interalia, slag as cementitious materials, as shown in Tragesser, U.S. Pat.No. 3,557,876 (Jan. 26, 1971).

SUMMARY OF THE INVENTION

It is an object of this invention to cement a slim hole well.

It is a further object of this invention to avoid the formation of voidsin a cementing job in a slim hole well.

It is a further object of this invention to avoid the problems ofincompatibility between filter cake and cement in the annulus of a slimhole well.

In accordance with this invention a slim hole borehole is drilledutilizing a drilling fluid comprising blast furnace slag and water, thedrilling fluid being circulated down a drill string and up an annulusbetween the drill string and the borehole wall, thus laying down afilter cake on the walls of the borehole during the drilling; thereafterthe drill string is removed, casing inserted and an activator is addedto the drilling fluid and the resulting cementitious slurry iscirculated down into the casing and up into the annulus.

DETAILED DESCRIPTION OF THE INVENTION

It has been discovered that by utilizing blast furnace slag in thedrilling fluid, a compatible filter cake is laid down on the boreholewall and thus the filter cake turns into an asset rather than aliability. In addition, as will be discussed in detail hereinbelow,undisplaced drilling fluid in the narrow annulus is converted into astrong, hard, sealing material.

In preferred embodiments wherein the drilling fluid is activated byadditional blast furnace slag and accelerators, as will be discussedhereinbelow, the rheological properties of both the drilling fluid andthe cement can be optimized. Finally, the drilling fluid and cementproduced in accordance with this invention is tolerant, indeed isbenefitted by, the presence of brine. Since a major area where slim holewells are utilized is in offshore drilling where brine is inevitablypresent this is of special significance. Since drilling fluid used inaccordance with this invention becomes a part of the final cement, thefluid can be referred to as a universal fluid, i.e., is useful in thedrilling and cementing.

The term `slim hole drilling` is intended to encompass those techniqueswhere the initial borehole diameter is 3 to 9 inches, more generally 4to 6 inches. In such wells the annulus is only 0.1 to 1.5; generally0.25 to 1.25, more generally 0.5 to 0.75 inches. Reference to thedimension of the annulus means the difference between the radius to theinner surface of the borehole and the radius to the outer surface of thecasing.

DRILLING FLUID

The term `drilling fluid` as used herein means water or oil based fluidswhich contain water, blast furnace slag and at least one other additivesuch as viscosifiers, thinners, dissolved salts, solids from the drilledformations, solid weighting agents to increase the fluid density,formation stabilizers to inhibit deleterious interaction between thedrilling fluid and geologic formations, and additives to improve thelubricity of the drilling fluid.

Generally, the initial drilling fluid will be any one of the known typesof drilling fluids which has been combined with blast furnace slag.Suitable drilling fluids include those known in the art as water-basedmuds, fresh water mud, sea water mud, salt mud, brine mud, lime mud,gypsum mud, polyalcohol mud and oil-in-water emulsions. Also,oil-containing muds which also contain water can be utilized such as lowwater content oil-base mud and invert oil-emulsion mud. One group ofsuitable drilling fluids is disclosed in Hale and Cowan, U.S. Pat. No.5,058,679 (Oct. 22, 1991), the disclosure of which is incorporatedherein by reference. In all cases the mud will also have blast furnaceslag which will generally be present in an amount within the range of 1to 100 lbs/bbl of final drilling fluid, preferably 10 to 80 lbs/bbl,most preferably 20 to 50 lbs/bbl.

A typical mud formulation to which slag may be added to form drillingfluid is as follows: 10-20 wt % salt based on total fluid weight, 8-10lbs/bbl bentonite, 4-6 lbs/bbl carboxymethyl starch (fluid losspreventor), sold under the trade name "BIOLOSE" by Milpark, 0.5-1lbs/bbl partially hydrolyzed polyacrylamide (PHPA) which is a shalestabilizer, sold under the trade name "NEWDRIL" by Milpark, 1-1.25lbs/bbl carboxy methyl cellulose (CMC) sold under the trade name"MILPAC" by Milpark, 30-70 lbs/bbl drill solids, and 0-250 lbs/bblbarite.

Because the drilling fluid becomes a part of the final cementitiousslurry, the amount of used drilling fluid which must be disposed of isgreatly diminished.

As noted hereinabove, both fresh and salt water muds can be utilized.There is, however, a preference for drilling fluids containing 0.1 to 26wt %, most preferably 3 to 10 wt % sodium chloride. One suitable sourcefor this is simply to use sea water or a brine solution simulating seawater. Contrary to what would be expected, the brine actually enhancesthe final strength of the cement.

Various salts, preferably inorganic salts, are suitable for use in thedrilling fluid used in this invention including, but not limited to,NaCl, NaBr, KCl, CaCl₂, NanO₃, NaC₂ H₃ O₂, KC₂ H₄ O₂, NaCHO₂, and KCHO₂among which sodium chloride is preferred, as noted above. Broadly, suchsalts can be used, if desired up to the saturation point under theconditions employed.

In another embodiment of this invention, most or all of the componentsof the drilling fluid are chosen such that they have a function in thecementitious material also. The following Table illustrates theuniqueness of such formulations.

                  TABLE A                                                         ______________________________________                                               Function                                                                      Drilling Fluid                                                                              Cementitious Slurry                                                           Secon-           Secon-                                  Additive Primary     dary    Primary  dary                                    ______________________________________                                        Synthetic                                                                              Fluid loss          Fluid loss                                                                             Re-                                     polymer.sup.1                                                                          control             control  tarder                                  Starch.sup.2                                                                           Fluid loss  Vis-    Fluid loss                                                                             Re-                                              control     cosity  control  tarder                                  Biopolymer.sup.3                                                                       Viscosity           Viscosity                                                                              Re-                                                                           tarder                                  Silicate Viscosity   Shale   Accelerator                                                                             --                                                          stabi-                                                                        lizer                                                    Carbohydrate                                                                           Deflocculant                                                                               --     Retarder Defloc-                                 polymer.sup.4                         culant                                  Barite.sup.5                                                                           Density      --     Density  Solids                                  Bentonite.sup.6                                                                        Fluid loss   --     Fluid loss                                                                             Solids                                           control             control                                          Clay/Quartz                                                                             --          --     Solids    --                                     dust.sup.7                                                                    Slag.sup.8                                                                             Cuttings     --     Cementitious                                                                           Solids                                           stabilizer                                                           Lime.sup.9                                                                             Cuttings/well-                                                                            Alka-   Accelerator                                                                            Solids                                           bore        linity                                                            stabilizer                                                           PECP.sup.10                                                                            Shale       Fluid   Retarder Rheo-                                   polyalcohol                                                                            stabilizer  loss             logical                                                                       control                                 NaCl     Shale        --      --       --                                              stabilizer                                                           ______________________________________                                         .sup.1 A synthetic polymer manufactured by SKW Chemicals Inc. under trade     name "POLYDRILL", for instance.                                               .sup.2 Starch made by Milpark Inc. under the trade name "PERMALOSE", for      instance.                                                                     .sup.3 "BIOZAN", a biopolymer made by Kelco Oil Field Group, Inc., for        instance.                                                                     .sup.4 A watersoluble carbohydrate polymer manufactured by Grain              Processing Co. under trade name "MORREX", for instance.                       .sup.5 Barite is BaSO.sub.4, a drilling fluid weighting agent.                .sup.6 Bentonite is clay or colloidal clay thickening agent.                  .sup.7 A clay/quartz solid dust manufactured by MilWhite Corp. under the      trade name "REVDUST", for instance.                                           .sup.8 Blast furnace slag manufactured by Blue Circle Cement Co. under th     trade name "NEWCEM" is suitable.                                              .sup.9 CaO                                                                    .sup.10 Polycyclicpolyetherpolyol                                        

The material in the above Table A labeled PECP is of specialsignificance in connection with this invention. This refers to apolyhydric alcohol most preferably a polycyclicpolyetherpolyol. Ageneral chemical composition formula representative of one class ofthese materials is as follows: ##STR1## where x≧1 and y≧0.

A more complete description of these polycyclicpolyetherpolyols is foundin Hale and Cowan, U.S. Pat. No. 5,058,679 (Oct. 22, 1991), thedisclosure of which is incorporated herein by reference.

SLAG

By `blast furnace slag` is meant the hydraulic refuse from the meltingof metals or reduction of ores in a furnace as disclosed in said Haleand Cowan, U.S. Pat. No. 5,058,679 (Oct. 22, 1991), the disclosure ofwhich is incorporated herein by reference.

The preferred blast furnace slag used in this invention is a high glasscontent slag produced by quickly quenching a molten stream of slag at atemperature of between 1,400° C. and 1,600° C. through intimate contactwith large volumes of water to convert the stream into a material in aglassy state having hydraulic properties. At this stage it is generallya granular material that can be easily ground to the desired degree offineness. Silicon dioxides, aluminum oxides, iron oxides, calcium oxide,magnesium oxide, sodium oxide, potassium oxide, and sulphur are some ofthe chemical components in slags. Preferably, the blast furnace slagused in this invention has a particle size such that it exhibits aBlaine specific surface area between 2,000 cm² /g and 15,000 cm² /g andmore preferably between 3,000 cm² /g and 15,000 cm² /g, even morepreferably, between 4,000 cm² /g and 9,000 cm² /g, most preferablybetween 4,000 cm² /g and 8,500 cm² /g. An available blast furnace slagwhich fulfills these requirements is marketed under the trade name"NEWCEM" by the Blue Circle Atlantic Company. This slag is obtained fromthe Bethlehem Steel Corporation blast furnace at Sparrow's Point, Md.

A usual blast furnace slag composition range in weight percent is: SiO₂,30-40; Al₂ O₃, 8-18; CaO, 35-50; MgO, 0-15; iron oxides, 0-1; S, 0-2 andmanganese oxides, 0-2. A typical example is: SiO₂, 36.4; Al₂ O₃, 16.0;CaO, 43.3; MgO, 3.5; iron oxides, 0.3; S, 0.5; and MnO₂ O₃ <0.1.

Blast furnace slag having relatively small particle size is frequentlydesirable because of the greater strength it imparts in many instancesto a final cement. Characterized in terms of particle size the term"fine" can be used to describe particles in the range of 4,000 to 7,000cm² /g. Corresponding to 16 to 31 microns in size; "microfine" can beused to describe those particles in the 7,000 to 10,000 cm² /g rangethat correspond to particles of 5.5-16 microns in size and "ultrafine"can be used to describe particles over 10,000 cm² /g that correspond toparticles 5.5 microns and smaller in size. In each instance, the surfacearea is Blaine specific surface area. Finely ground blast furnace slagis available from Blue Circle Cement Company under the trade name"NEWCEM", from Geochem under the trade name "MICROFINE MC-100" and fromKoch Industries of Tulsa, Okla. under the trade name "WELL-CEM". TheKoch product has a Blaine specific surface area of 10,040 cm² /gm.

However, it is very time consuming to grind blast furnace slag to theseparticle sizes. It is not possible to grind blast furnace slag in amanner where particles are entirely one size. Thus, any grindingoperation will give a polydispersed particle size distribution. A plotof particle size versus percent of particles having that size would thusgive a curve showing the particle size distribution.

In accordance with a preferred embodiment of this invention a blastfurnace slag having a polydispersed particle size distributionexhibiting at least two nodes on a plot of particle size versus percentof particles in that size is utilized. It has been found that if only aportion of the particles are in the ultrafine category, the remaining,indeed the majority, of the slag can be ground more coarsely and stillgive essentially the same result as is obtained from the more expensivegrinding of all of the blast furnace slag to an ultrafine state. Thus, agrinding process which will give at least 5% of its particles fallingwithin a size range of 1.9 to 5.5 microns offers a particular advantagein economy and effectiveness. More preferably, 6 to 25 wt % would fallwithin the 1.9 to 5.5 micron range. The most straightforward way ofobtaining such a composition is simply to grind a minor portion of theblast furnace slag to an ultrafine condition and mix the resultingpowder with slag ground under less severe conditions. Even with the lesssevere conditions there would be some particles within the microfine orultrafine range. Thus, only a minority, i.e., as little as 4 wt % of theslag, would need to be ground to the ultrafine particle size. Generally,5 to 20%, more preferably 5 to 8% can be ground to the ultrafineparticle size; the remainder can be ground in a normal way, thus givingparticles generally in a size range of greater than 11 microns, themajority being in the 11 to 31 micron range.

Another feature of this invention is the ability to tailor the rheologyof both the drilling fluid and the final cement to the conditions of aparticular wellbore. This results from the fact that the use of slag asthe hydraulic material gives a final cementitious slurry which is notweakened in the manner that would be the case with Portland cement ifthe slurry is more dilute. On the other hand, additional slag does notimpart extremely high viscosity to the slurry and thus a higherconcentration of hydraulic material can be used if desired.

DILUTION

However, in the preferred method of this invention, the drilling fluidis utilized and thereafter diluted prior to or during the addition ofadditional blast furnace slag. The dilution fluid can be the same as theliquid used to make the drilling fluid or it can be different.Generally, it will be brine, especially if the drilling fluid was madeusing brine. It can also be a more concentrated brine. In manyinstances, it is preferred that both the dilution fluid and the originalliquid used to produce the initial drilling fluid be seawater. This isespecially beneficial in offshore drilling applications where freshwater is not readily available and seawater is.

Thus, a significant improvement in the operating procedure is provided.This is because the density of the drilling fluid can be chosen in thefirst place to be sufficient to avoid inflow into the wellbore becauseof formation pressure but insufficient to rupture the wellbore wall andforce fluid out into the formation. By utilizing the dilution andthereafter the addition of additional blast furnace slag, thecementitious slurry can also have the density tailored to the particularoperation the same as the drilling fluid.

The dilution can be carried out in either of two ways. First, a vesselcontaining drilling fluid can simply be isolated and the desired amountof water or other diluent added thereto. In a preferred embodiment,however, the drilling fluid is passed to a mixing zone as a flowingstream and the dilution fluid added "on the fly" to the flowing stream.Thereafter, the additional blast furnace slag is added. This avoidshighly viscous cementitious slurry compositions and allows all of thepumping to be done with piping and pumps associated with the well rigwithout the need for pumps designed for pumping cement. This is ofparticular value in the areas to which this invention is of specialutility, offshore drilling rigs where the transportation of additionalpumping equipment is particularly inconvenient. Thus, it is possible totailor the final density of the cementitious slurry, if desired, to avalue within the range of 30% less to 70% more than the original densityof the drilling fluid, preferably within the range of 15% less to 50%more, most preferably essentially the same, i.e., varying by no morethan ±5 weight percent.

The total amount of cementitious material present in the cementitiousslurry generally ranges from about 20 lbs/bbl to about 600 lbs/bbl,preferably 100 lbs/bbl to 500 lbs/bbl, most preferably 150 lbs/bbl to350 lbs/bbl. In a most preferred embodiment the hydraulic material ismade up entirely, or essentially entirely, of blast furnace slag, noother hydraulic material being added.

In yet another related embodiment of this invention, universal fluid isutilized in a drilling operation and thereafter additional slag and/oradditives are gradually added so as to gradually transition from adrilling fluid to a cementitious slurry.

This invention can even be used to drill extended reach slim hole wellswhich, themselves, present some of the same problems as slim hole wellsgenerally because of uneven casing placement in the borehole.

The term `extended reach drilling` is intended to encompass thosetechniques sometimes referred to as deviated drilling, horizontaldrilling or directional drilling. The invention is applicable to thosedeviated wells where the deviation from vertical is as little as onepercent since even this small deviation over the course of a wellborecauses difficulty in centering the casings. The other extreme would be atruly horizontal well which would be quite rare. The invention is ofparticular value in those wells having a deviation from vertical withinthe range of 1 to 90 degrees, preferably 10 to 90 degrees, mostpreferably 30 to 80 degrees.

MIXED METAL HYDROXIDES

In some instances, it is desirable to sequence the incorporation ofingredients. For instance, the slag may be introduced into the drillingfluid after the addition of thinners and/or retarders. This isparticularly true if mixed metal hydroxides are used in the drillingfluid to impart thixotropic properties. In such instances, a thinnersuch as a lignosulfonate is preferably added before adding slag.

The mixed metal hydroxides provide better solids suspension. This, incombination with the settable filter cake provided in the technique ofthis invention, greatly enhances the cementing in a restricted annulus.The mixed metal hydroxides are particularly effective in muds containingclay such as sodium bentonite. Preferred systems thickened in this waycontain from 1 to 20 lbs/bbl of clay such as bentonite, preferably 2 to15 lbs/bbl, most preferably 7 to 12 lbs/bbl. The mixed metal hydroxidesare generally present in an amount within the range of 0.1 to 2 lbs/bblof total drilling fluid, preferably 0.1 to 1.5 lbs/bbl, most preferably0.7 to 1.2 lbs/bbl. Mixed metal hydroxides are known in the art and aretrivalent metal hydroxide-containing compositions such as MgAl(OH)₄.7Cl₀.3. They conform essentially to the formula

    Li.sub.m D.sub.d T(OH).sub.(m+2d+3+na) A'.sub.a n

where

m represents the number of Li ions present; the said amount being in therange of zero to about 1;

D represents divalent metals ions; with

d representing the amount of D ions in the range of zero to about 4;

T represents trivalent metal ions;

A" represents monovalent or polyvalent anions of valence -n, other thanOH⁻, with a being the amount of A' anions; and where (m+2d+3+na) isequal to or greater than 3.

A more detailed description can be found in Burba, U.S. Pat. No.4,664,843 (May 12, 1987). The mixed metal hydroxides in the drillingfluid in combination with blast furnace slag tend to set to a cementhaving considerable strength in a comparatively short time, i.e., aboutone-half hour at temperatures as low as 100° F. This can be a majorasset in some applications.

CONVENTIONAL DRILLING FLUID ADDITIVES

Suitable fluid loss additives found in drilling fluids include bentoniteclay, carboxymethylated starches, starches, carboxymethyl cellulose,synthetic resins such as "POLYDRILL" by SKW Chemicals, sulfonatedlignite, lignites, lignin, or tannin compounds. Weight materials includebarite, calcium carbonate, hematite and MgO, for example. Shalestabilizers that are used in drilling fluids include hydrolyzedpolyacrylonitrile, partially hydrolyzed polyacrylamide, salts includingNacl, KCl, sodium or potassium formate, sodium or potassium acetate,polyethers and polycyclic and/or polyalcohols. Viscosifying additivescan be used such as biopolymers, starches, attapulgite and sepiolite.Additives are also used to reduce torque. Suitable thinners such aschrome and chrome free lignosulfonates, sulfonated styrenemaleic-anhydride and polyacrylate may also be used depending upon themud type and mud weight. Lubricating additives include nonionicdetergents and oil (diesel, mineral oil, vegetable oil, synthetic oil),for instance. Alkalinity control can be obtained with KOH, NaOH or CaO,for instance. In addition, other additives such as corrosion inhibitors,nut hulls etc. may be found in a typical drilling fluid. Of course,drill solids including such minerals as quartz and clay minerals(smectite, illite, chlorite, kaolinite, etc.) may be found in a typicalmud.

ACTIVATION

After the completion of the drilling, the drilling fluid is activated.This can be done by adding activators as discussed in detailhereinbelow, by adding additional blast furnace slag, or both.Preferably, the activation is done by adding additional blast furnaceslag and an accelerator. It is also within the scope of the inventionfor the cementitious slurry to comprise hydraulic materials other thanblast furnace slag, for instance, pozzolans and other hydraulicmaterials can be present. By `hydraulic material` is meant a materialwhich, on contact with water and/or activators, hardens or sets into asolidified composition. The activator or activators can be added eitherwith any other ingredients that are added before the additional blastfurnace slag, with the additional blast furnace slag, or after theaddition of the additional blast furnace slag.

In some instances, it may be desirable to use a material which functionsas a retarder along with the activator because of the need for othereffects brought about by the retarder. For instance, a chromiumlignosulfonate may be used as a thinner along with the activator eventhough it also functions as a retarder.

As noted hereinabove, the activator can be nothing more than additionalblast furnace slag in a concentration high enough to form a cementitiousslurry. Blast furnace slag will eventually hydrolyze and form cementparticularly if it is in a heated environment or if heat is applied tothe wellbore to speed up the setting.

However, it is greatly preferred to utilize a combination of slag in anamount which represents the majority of the total slag in thecementitious composition (i.e., a majority of the total of the slag inthe drilling fluid and the slag added to form the cementitious slurry)and a chemical activator. Suitable chemical activators include sodiumsilicate, sodium fluoride, sodium silicofluoride, magnesiumsilicofluoride, zinc silicofluoride, sodium carbonate, potassiumcarbonate, sodium hydroxide, potassium hydroxide, calcium hydroxide,sodium sulfate and mixtures thereof. A mixture of caustic soda (sodiumhydroxide) and soda ash (sodium carbonate) is preferred because of theeffectiveness and ready availability. When mixtures of alkaline agentssuch as caustic soda and soda ash are used the ratio can vary ratherwidely since each will function as an accelerator alone. Preferably,about 1 to 20 lbs/bbl of caustic soda, more preferably 2 to 6 lbs/bbl ofcaustic soda are used in conjunction with from 2 to 50 lbs/bbl,preferably 7 to 21 lbs/bbl of soda ash. The references to "lbs/bbl"means pounds per barrel of final cementitious slurry.

FILTER CAKE SETTING

In yet another embodiment of this invention the drilling process iscarried out as described hereinabove with a universal fluid to produce aborehole through a plurality of strata, thus laying down a filter cake.Prior to the cementing operation, an activator is passed into contactwith the filter cake, for instance by circulating the activator down thedrill string and up the annulus between the drill string and the filtercake, or else the drill string is removed and the casing inserted andthe activator circulated down the casing and up the annulus. As usedherein `down` as it relates to a drill string or casing means in adirection toward the farthest reach of the borehole even though in rareinstances the borehole can be disposed in a horizontal position.Similarly, `up` means back toward the beginning of the borehole.Preferably, the circulation is carried out by using the drill string,this being the benefit of this embodiment of the invention whereby thefilter cake can be "set" to shut off gas zones, water loss, or to shutoff lost circulation in order to keep drilling without having to removethe drill string and set another string of casing. This can also be usedto stabilize zones which may be easily washed-out (salt zones whereinthe salt is soluble in water, for instance) or other unstable zones.After the drilling is complete the drilling fluid is then diluted, thedrill string removed, and the cementing carried out as describedhereinabove. This can be accomplished by circulating a separate fluidcontaining the activator or by adding an activator such as an alkali asdescribed hereinabove to the drilling fluid.

Conventional spacers may be used in the above described sequence. Also,any leftover fluid having activators therein may be displaced out of theborehole by the next fluid and/or a spacer fluid and stored forsubsequent use or disposal.

In this embodiment where the filter cake is "set", the activator can beany of the alkaline activators referred to hereinabove such as a mixtureof sodium hydroxide and sodium carbonate.

EXAMPLE

A 13.9 lb/gal universal fluid (UF) was prepared using a 13.5 lb/gal mudhaving the following composition: 20 wt % salt (140,000 mg/l), 8-10lbs/bbl bentonite, 4-6 lbs/bbl carboxymethyl starch, sold under thetrade name "BIOLOSE" by Milpark, 0.5-1 lbs/bbl partially hydrolyzedpolyacrylamide (PHPA), sold under the trade name "NEWDRIL", by Milpark,1-1.25 lbs/bbl carboxymethyl cellulose (CMC), sold under the trade name"MILPAC" by Milpark, 30-70 lbs/bbl drill solids, 0-250 lbs/bbl barite,and 40 lbs/bbl of blast furnace slag sold under the trade name "NEWCEM"by Blue Circle Atlantic Company.

The universal fluid was designed to be a drilling fluid at temperatures120° F. through 160° F. and to provide a settable filter cake for betterzonal isolation and for protection of the formation. A full-scalehorizontal wellbore model was used to test the hardening of thisuniversal fluid. This universal fluid was tested to show that it and itsfilter cake would set up under downhole conditions.

The 13.9 lbs/gal universal fluid was circulated through the model and afilter cake was formed. A portion of the UF was then converted into a15.4 lb/gal cement slurry by the addition of slag and activators asdescribed hereinafter and the cement slurry was used to displace theuniversal fluid (UF) in the wellbore model. The wellbore model was heataged at 200° F. for three weeks. The most important objective of thistest was to deposit the UF in simulated washed-out sections and to showthat undisplaced UF pockets can be set up in the worst possible physicalconditions.

Test Objectives were: 1) Demonstrate that undisplaced pockets of the13.9 lb/gal UF can set up under downhole conditions in order toeliminate unset fluids and filter cakes in the test model, 2)Demonstrate that the 13.9 lb/gal UF could be converted into a cementslurry with satisfactory slurry and set cement properties, 3)Demonstrate that a UF/cement slurry job can improve zonal isolation(improved shear and hydraulic bond in the model) and provide lateralcasing support.

Test Conditions: Three washed-out sections (1", 2" and 6" wide) werespecially built by modifying a synthetic wellbore model. Thedisplacement test on the modified model was run based on the followingfield conditions:

UF Condition--Drilling a 105/8 in. deviated hole at borehole circulatingtemperature (BHCT) of 120° F. to 160° F.

Cement slurry Condition--Borehole static temperature (BHST) of 200° F.(18,000 ft.)

Displacement Model: A 5-inch outside diameter (OD), 18-ft long steelcasing in a 61/2-inch inside diameter (ID) synthetic core simulating aformation with the 3 washed-out sections was used in a horizontalposition using the following test conditions: 1) slow displacement rates(1 bpm) in order not to wash away the deposited filter cake and topreserve undisplaced UF in the washed-out sections, 2) 100% casingstandoff.

The synthetic core (formation) was a 13/4-inch thick layer of apermeable sand-epoxy mix on the inner circumference of the model(103/4-in. OD and 10-in. ID steel casing). Three washed out sections(sharp edged with no transition zones) were made by removing portions ofthe simulated sand-epoxy formation before welding the sections of the15-ft total length steel casing together. The tops of the threewash-outs (1", 2" and 6" length) were placed at 5 feet, 7 feet, and 10feet respectively from the bottom of the wellbore model. As mentionedearlier, the 5-in. OD steel casing was centralized inside the wellboremodel leaving a theoretical annular clearance of 3/4-inch except in thewash-outs where it was 21/2-inch wide.

The 3/4-inch annular clearance simulates the narrow annulus in a slimhole well.

Deposition of filter cake: The above-described 13.9 lb/gal universalfluid was circulated for 2 hours through the water-saturated wellboremodel at 3-4 bbl/min. The wellbore model was shut in, electricallyheated to 140° F., and pressurized. Filtrate was collected at a pressuredifferential of 100 psi to build a thick filter cake approximately1/8th-inch thick on the core.

Displacement: After the overnight filtering period at 140° F., the UFwas circulated through the wellbore model at 1 bpm for 20 minutes whilecollecting additional filtrate. During the time of circulation, a 5-bblbatch of UF was isolated from the active mud (UF) system and convertedit into a 200 lb/bbl cement slurry by adding additional 160 lb/bbl"NEWCEM" brand slag, activators and a retarder. The cement slurry wasthen dyed by adding 2.5 lb/bbl red iron oxide. The activator system wasmade up of 4 lb/bbl caustic soda, 14 lb/bbl soda ash and 6 lb/bbl"SPERCENE CF" (chrome lignosulfonate manufactured by M-I DrillingFluids). The UF was displaced out of the wellbore model with the dyedcement slurry at 1-2 bbl/min until the initial portion of uncontaminatedcement slurry was noted at the discharge. The slow displacement was notsought in order to leave substantial amounts of undisplaced UF in themodel. The cement slurry was then circulated through the wellbore modelfor an additional 20 minutes at 1-2 bbl/min.

Evaluation of Core: After a three-week aging period at 200° F., thewellbore model was allowed to cool to room temperature. The model wasthen disassembled and sawed into three sections. These sections wereagain sawed into smaller sections for further shear and hydraulic bondtests.

All the available cross sections were photographed for estimatingdisplacement efficiency. As planned, the displacement efficiency wasabout 55%. The expected poor displacement efficiency of the washed-outmodel was accentuated by the high angle of the wellbore model. Althoughthe casing was centralized, a perfect centralization was not obtained.Much of the UF was not displaced out of the narrow side of the annulus.By `displacement efficiency` is meant the volume of mud removed dividedby the volume of the annulus times 100 to convert to percent.

Thus, this example demonstrates the value of the invention in a narrowannulus such as is found in a slim hole well.

The presence of undisplaced UF was evident all the way from one end tothe other end, especially on the bottom side. Even so, the UF filtercakes and undisplaced UF pockets were found to be very hard. A hand heldpenetrometer test indicates that the UF filter cakes and cement slurryhad compressive strengths in excess of 750 psi (maximum reading). Inessence, an excellent cement job was obtained in spite of the slowpumping rates, high angle (90°) of the hole and presence of the threewash-outs. The cement job would have been very poor with a displacementefficiency of 55% if the universal fluid/cement slurry had not beenused.

Blocks containing the washed-out portions of the model were sawed andfurther evaluated. Lengthwise diamond saw cuts were made to produceparallel faced slices about 11/4-inch to 13/4-inch thick. These slicesexposed the newly cut cross sections of the formation, the annulus, thewash-outs and the hardened UF, UF filter cake, and the cement slurry.

Hardened UF and UF filter cake were found in the 1-inch wash-out, at thebase of the 2-inch wash-out, and at the base corners of the 6-inchwash-out. The increased hardness near the permeable formation faces inthe wash-outs is caused by the concentration of the UF by fluid loss.The hardened universal fluids showed a compressive strength betweenabout 500 and 1,500 psi. Although there were various lamination in thesamples, the annulus and washed-out sections were completely cementedand the overall zonal isolation was excellent.

Additionally, 8 slag cement slurry samples were taken during thedisplacement and cured in 2-inch cube molds at 200° F. for one week. Anaverage compressive strength of 1,874 psi with a standard deviation of204 psi was obtained.

Hydraulic Bond Test Results: The top section (4.5-ft long) was cut intotwo smaller sections. In-situ hydraulic bond tests were conducted onthese cores (2.25-ft long) using a fluorescent dyed water. Two taps(front and back) were drilled to the casing on the core and nipples wereinstalled using epoxy resin. Dyed water was pumped through the nipplesusing an hydraulic pump and maximum breakdown pressures were recorded ashydraulic bond strength. The test results were as follows:

    ______________________________________                                        Core/Pressure Tap                                                                            Hydraulic Bond, psi                                            ______________________________________                                        H-1,      Front    1,800                                                      H-1,      Back     400                                                        H-2,      Front    550                                                        H-2,      Back     750                                                        ______________________________________                                    

Shear Bond Test Results: The two bottom sections of the model (4.5-ftlong) were cut into 5 pieces for shear bond tests. The sectionscontaining the wash-outs were excluded from the shear bond tests. Shearbond was measured by pressing out the casing on a hydraulic press. Thetest results were as follows:

    ______________________________________                                        Sample ID                                                                             Pipe Length (in.)                                                                          Force (lbs)                                                                              Shear Bond (psi)                              ______________________________________                                        S-1     10.175       4,140      26.2                                          S-2     9.0          19,200     135.38                                        S-3     8.5          1,700      12.7*                                         S-4     10.5         10,200     60.8                                          S-5     10.675       5,360      32.0                                          S-6     10.875       4,960      29.1                                          S-7     10.75        1,580      9.4*                                          S-8     8.675        2,000      14.7*                                         ______________________________________                                         *Specimens appeared damaged during sawing the core.                      

In addition, 4 slag cement slurry samples were taken during thedisplacement and cured in laboratory shear bond molds with a steel pipehaving a dimension of 4-in. length and 1.5-in. OD at 200° F. for oneweek. An average shear bond was 46.5 psi.

Conclusions: The undisplaced pockets of the universal fluid were sethard with a compressive strength between 500 and 1,500 psi. Excellenthydraulic bond data were obtained. Although varied, shear bond data arevery good. It is demonstrated that (1) a blast furnace slag universalfluid can achieve a 100% displacement efficiency by eliminating unsetpockets of mud even in a narrow annulus, (2) a blast furnace slaguniversal fluid can improve a lateral support of casing by eliminatingunset materials and providing additional strength through solidificationof those portions which otherwise would be unset, (3) a blast furnaceslag universal fluid can improve zonal isolation through improving shearand hydraulic bonds, and (4) a high density blast furnace slag universalfluid can be formulated and utilized at elevated temperatures.

Two UCA cells with the 15.4 lb/gal slag cement slurry sample caughtduring the displacement were run at 200° F. Additional tests were runwith the slag cement slurry sample to obtain rheology, density, APIfluid loss, and free water data. The test results of the 15.4 lb/galslag cement slurry are summarized in the following table.

    ______________________________________                                        FORMULATION:                                                                  1 bbl Mud + 4 lbs NaOH + 14 lbs Na.sub.2 CO.sub.3 + 8                         lbs "SPERCENE" brand chromium lignosulfonate +                                200 lbs "NEWCEM" brand blast furnace slag                                     ______________________________________                                        SLAG CEMENT SLURRY PROPERTIES:                                                Plastic Viscosity =     14 cp                                                 Yield Point =           10 lb/100 ft..sup.2                                   Gel Strength, 10 sec/10 min =                                                                         8/36 lb/100 ft..sup.2                                 API Fluid Loss =        4.9 cm.sup.3                                          High Temperature Fluid Loss =                                                                         24 cm.sup.3 at 160° F.                         Free Water =            0 cm.sup.3                                            Thickening Time =       7:43 hrs at 170° F.                            SLAG CEMENT SET STRENGTHS after CURING AT                                     200° F.                                                                Compressive Strength =  2,200 + psi                                           Shear Bond =            30-135 psi                                            Hydraulic Bond =        400-1,800 psi                                         ______________________________________                                    

While this invention has been described in detail for the purpose ofillustration, it is not to be construed as limited thereby but isintended to cover all changes and modifications within the spirit andscope thereof.

What is claimed is:
 1. A method comprising:drilling a slim hole wellborewith a drill string comprising a drill pipe utilizing a drilling fluidcomprising granulated water-quenched blast furnace slag and water;circulating said drilling fluid down said drill pipe and up an annulusbetween said drill pipe and walls of said wellbore, thus laying down afilter cake on said walls of said wellbore during said drilling;withdrawing said drill string and inserting a casing, thus creating anannulus having a dimension between 0.1 and 1.25 inches between saidcasing and said walls of said wellbore; adding an activator to saiddrilling fluid to produce a cementitious slurry; and circulating saidcementitious slurry down said casing and up into said annulus betweensaid casing and said walls of said wellbore.
 2. A method according toclaim 1 wherein said activator is additional blast furnace slag andalkaline agent.
 3. A method according to claim 2 wherein said alkalineagent is a mixture of caustic soda and soda ash.
 4. A method accordingto claim 3 wherein said drilling fluid comprises, in addition, alignosulfonate thinner.
 5. A method according to claim 2 wherein saiddrilling fluid contains from 20 to 50 lbs/bbl of said blast furnace slagand wherein said additional blast furnace slag is added in forming saidcementitious slurry in an amount sufficient to give a total amount ofcementitious material in said cementitious slurry within the range of100 to 500 lbs/bbl.
 6. A method according to claim 5 wherein said blastfurnace slag represents the only hydraulic component in saidcementitious slurry.
 7. A method according to claim 2 wherein said slagof said drilling fluid and said slag of said activator each are made upof a mixture of 5-25wt % ultrafine ground slag and 80-95wt % of fine ormicrofine ground slag.
 8. A method according to claim 1 wherein saidwater contains dissolved salts.
 9. A method according to claim 8 whereinsaid dissolved salts comprise sodium chloride.
 10. A method according toclaim 9 wherein said sodium chloride is present in an amount within therange of 0.2 to 5 wt %.
 11. A method according to claim 8 wherein saidwater comprises sea water.
 12. A method according to claim 1 whereinsaid drilling fluid contains, in addition, a polycyclicpolyetherpolyol.13. A method according to claim 1 wherein said borehole has a diameterwithin the range of 4 to 6 inches and said annulus has a dimensionwithin the range of 0.25 to 1.25 inches.
 14. A method according to claim1 wherein said drilling fluid contains clay and a mixed metal hydroxidethixotropic agent.
 15. A method according to claim 14 wherein said clayis bentonite and said mixed metal hydroxide is MgAl(OH)₄.7 Cl₀.3,wherein a lignosulfonate thinning agent is added prior to adding saidactivator, and wherein said activator comprises additional blast furnaceslag and an alkaline agent.
 16. A method according to claim 1wherein:prior to said withdrawing of said drill string, an activator ispassed down said drill pipe and up into contact with said filter cake,thus causing setting of said filter cake; and thereafter additionaldrilling is carried out.
 17. A method for drilling a slim hole wellcomprising:drilling a borehole with a rotary drill comprising a drillpipe and a bit; utilizing a drilling fluid comprising 20-50 lbs/bbl ofgranulated water-quenched blast furnace slag, clay, starch, partiallyhydrolyzed polyacrylamide, and barite; circulating said drilling fluiddown said drill pipe and up an annulus between said pipe and the wallsof said borehole thus laying down a filter cake on said walls of saidborehole; withdrawing said rotary drill and inserting a casing, thuscreating an annulus between said casing and said walls of said boreholehaving a dimension between 0.1 and 1.25 inches; adding blast furnaceslag, a lignosulfonate thinner, and an activator comprising a mixture ofcaustic soda and soda ash to said drilling fluid to produce acementitious slurry having a total amount of blast furnace slag withinthe range of 100 to 500 lbs/bbl; and passing said cementitious slurrydown through said casing and up into said annulus formed by said casingand said walls of said borehole.